1. Field of the Invention
The present invention relates to a novel compound and organic light emitting diode using the same, particularly to m-terphenyl derivatives and organic light emitting diode using the same.
2. Description of the Prior Art
OLED works on the principal that electrons and holes diffuse through an electron transport layer (ETL) and hole transport layer (HTL), respectively, to enter a light-emitting layer, and recombine in the emitting region to form a particle generally referred as exciton. In order for the exciton to relax to the ground state, the energy is given off in the form of photo radiation. The radiation color can be tuned by applying different emitting materials. OLED has been highly-regarded due to a lot of advantages, such as self illumination, wider visual angle (>170°), shorter response time (˜μs), higher contrast, higher efficiency, lower power consumption, higher brightness, lower operative voltage (3-10V), thinner size (<2 mm), flexibility and so on.
The exciton generated from recombining holes and electrons may have triplet state or singlet state for its spin state. The singlet exciton relaxation would radiate fluorescence, and the triplet exciton relaxation would radiate phosphorescence. Phosphorescence achieves 3-fold efficiency when compared to fluorescence and may greatly enhance the IQE (internal quantum efficiency) of devices up to 100% by adopting heavy metal in electroluminescent configuration to achieve strong spin-orbital coupling and mixing of singlets and triplets. Therefore, phosphorescent heavy metals are now adopted as phosphorescent dopants in the emitting layer of OLED. In addition, by applying a doping method on the emitting layer, self-quenching of the emitting materials can be reduced greatly to enhance the efficiency of the device.
However, due to the higher triplet energy level for phosphorescent molecules, some energy would tend to flow to other material with lower triplet energy level and cause illumination in the device. Therefore, A good electron transport material should include following properties: 1. reversible electrochemical redox properties; 2. suitable HOMO and LUMO values so as to provide lowered operating voltage for good electron injection and optional hole blocking properties, where the HOMO value for the known electron transport material with hole blocking property is usually more than 6 eV; 3. higher electron mobility so as to obtain the combination area departed from the cathode and increase the generation rate of excitons; 4. higher thermal stability so as to prevent the heat generated by driving device from shortening the life expectancy of devices; and 5. good film forming ability.
To sum up, it is an important issue to achieve balance between electrons and holes in the devices and provide devices with better performance; therefore, it is necessary and critical to develop a novel carrier transport material.